In general, crystal oscillators are classified, by their functions, as SPXOs (Simple Packaged Crystal Oscillators), VCXO (Voltage Controlled Crystal Oscillators), TCXO (Temperature Compensated Crystal Oscillators), etc. An SPXO is a crystal oscillator that does not perform temperature compensation. A TCXO (temperature-compensated oscillator) is a crystal oscillator that performs temperature compensation so as not to change output frequency even if ambient temperature changes. A VCXO (voltage-controlled oscillator) is a crystal oscillator that changes output frequency in accordance with a control voltage that is supplied externally. A crystal oscillator with the function of VCXO added to TCXO is called a VC-TCXO. A VC-TCXO is used, for example, in a cellular phone in order to implement AFC (Approximately Frequency Control) function for the cellular phone to adjust frequency more accurately with reference to a signal from a base station.
Hitherto, manufacturers of oscillators have been developing these oscillators independently of each other in accordance with the market and customer needs in order to manufacture and supply oscillators that satisfy the customer needs.
However, since TCXOs, VCXOs, etc. have generally been developed independently of each other, and even common parts have been developed independently, efficiency of development has been inadequate. Furthermore, due to the independent development, separate production lines and separate inventory management of parts have been needed in spite of many parts being common.
In recent years, with the progress of information-oriented society, out of the need for improved processing ability and faster communication speed of electronic apparatuses, demand has been raised for a wider operating temperature range and reduction in phase noise of the oscillators.